Shank structure for rotary drill bits

ABSTRACT

A shank configuration for rotary drill bits is disclosed for positioning of the shank in relation to a bit body. A tapered surface or feature of the shank may be configured and sized to matingly engage a complementarily shaped surface or feature of the drill bit body and thereby become centered or positioned in relation thereto. A deformable element may be disposed between the shank and bit body. Also, the shank may comprise a material having a carbon equivalent of less than about 0.35%. A multi-pass weld procedure may be employed to affix the shank and bit body to one another wherein welds may be formed so that one weld originates at a circumferential position that differs from the origination circumferential position of its immediately preceding weld by at least about 90°. Further, a stress state may be developed within the multi-pass weld. A method of manufacture is also disclosed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.11/947,613, filed Nov. 29, 2007, now U.S. Pat. No. 7,594,454, issuedSep. 29, 2009, which is a divisional of U.S. patent application Ser. No.11/090,932, filed Mar. 25, 2005, now U.S. Pat. No. 7,472,764, issuedJan. 6, 2009, the disclosure of each of which is hereby incorporatedherein by reference in its entirety. This application is also related toU.S. patent application Ser. No. 11/947,624, filed Nov. 29, 2007, nowU.S. Pat. No. 7,600,589.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to a drill bit shank for rotarydrill bits for drilling subterranean formations and to rotary drill bitsso equipped.

2. State of the Art

A typical rotary drill bit includes a bit body secured to a hardenedsteel shank having a threaded pin connection for attaching the bit to adrill string, and a crown including a face region carrying cuttingstructures for cutting into an earth formation. Generally, if the bit isa fixed-cutter or so-called “drag” bit or drill bit, the cuttingstructures include a plurality of cutting elements formed at least inpart of a superabrasive material, such as polycrystalline diamond.Rotary drag bits employing polycrystalline diamond compact (PDC) cuttershave been employed for several decades. Typically, the bit body may beformed of steel, or a matrix of hard particulate material such astungsten carbide (WC) infiltrated with a binder, generally of a copperalloy.

In the case of steel body drill bits, the bit body may typically bemachined from round stock to a desired shape. Radially andlongitudinally extending blades, internal watercourses for delivery ofdrilling fluid to the bit face, and topographical features defined atprecise locations on the bit face may be machined into the bit bodyusing a computer-controlled, multi-axis machine tool. Hard-facing forresisting abrasion during drilling is usually applied to the bit faceand to other critical areas of the bit exterior, and cutting elementsare secured to the blades on the bit face, generally by inserting theproximal ends thereof into cutting element pockets machined therein.After machining and hardfacing, the bit body may be secured to ahardened steel shank having a threaded pin connection for securing thesteel body rotary drill bit to the drive shaft of a downhole motor ordirectly to drill collars at the distal end of a drill string rotated atthe surface by a rotary table or top drive.

Matrix-type drill bits, on the other hand, include a bit body formed ofa matrix of hard particulate material such as tungsten carbide containedwithin a graphite mold and infiltrated with a binder, generally of acopper alloy. Cast resin-coated sand, graphite displacements or, in someinstance, tungsten carbide particles in a flexible polymeric binder, maybe employed to define internal watercourses and passages for delivery ofdrilling fluid to the bit face, cutting element sockets or pockets,ridges, lands, nozzle apertures, junk slots and other externaltopographic features of the matrix-type rotary drag bit. However,because a matrix material comprising tungsten carbide or otherrelatively hard particles may be substantially unmachinable, amachinable steel blank is typically disposed within the bit mold priorto infiltration of the matrix material, the steel blank forming aportion of the matrix-type rotary drag bit body upon hardening of theinfiltrant that affixes the blank therein. In a manner similar tofabrication of steel body drill bits, the matrix-type bit body, via themachinable blank, may be secured to a hardened steel shank having athreaded pin connection for securing the bit to the drive shaft of adownhole motor or directly to drill collars at the distal end of a drillstring rotated at the surface by a rotary table or top drive.

Thus, in either steel body or matrix-type rotary drill bits, alignmentbetween the bit body and the hardened shank is critical because theshank, which includes the threaded pin connection, may determine theaxis of rotation of the bit. Alignment of the axis of rotation inrelation to the cutting element design is obviously of great importancein the operation of a rotary drag bit because aspects of the rotarydrill bit design may be based, at least in part, on cutting elementpositions as well as predicted forces thereon. For instance, so-called“anti-whirl” designs utilize a preferential lateral force directedtoward a pad designed to ride against the formation in order tostabilize the rotary drag bit. Conventionally, a threaded connection hasbeen employed between matrix-type bit bodies and the hardened shank, asdescribed in more detail hereinbelow.

FIGS. 1A and 1B illustrate a conventional matrix-type drill bit 10formed generally according to the description above. Conventionalmatrix-type drill bit 10 includes a central longitudinal axis 3 and bore12 therethrough for communicating drilling fluid to the face of the bitduring drilling operation. Cutting elements 5 and 7 (typically diamond,and most often a synthetic polycrystalline diamond compact or PDC) maybe bonded to the bit face during infiltration of the bit body ifthermally stable PDCs, commonly termed TSPs, are employed, or may besubsequently bonded thereto, as by brazing, adhesive bonding, ormechanical affixation.

The conventional preformed, so-called blank 14 comprising relativelyductile steel may also provide internal reinforcement of the bit bodymatrix 19. The blank 14 may be typically comprised of relatively ductilesteel because the high temperatures experienced by the blank duringinfiltration may generally anneal most steel materials. Blank 14 maycomprise a cylindrical or tubular shape, or may be fairly complex inconfiguration and include protrusions corresponding to blades, wings orother features on the bit face. The protrusions or fingers may begenerally welded into longitudinal slots formed within the tubularportion of blank 14. The blank 14 and other preforms as mentioned abovemay be placed at appropriate locations within the graphite mold used tocast the bit body. The blank 14 may be affixed to the bit body matrix 19upon cooling of the bit body after infiltration of the tungsten carbidewith the binder in a furnace, and the other preforms are removed oncethe matrix has cooled. Blank 14 may be machined and affixed to shank 16by way of threaded connection 15 as well as weld 20. Conventionally, acontinuous weld may be formed between shank 16 and blank 14. The shank16 typically is formed from an AISI 4140 steel, a material having acarbon equivalent of higher than about 0.35%, which requires the shank16 and blank 14 to be preheated prior to welding. Shank 16 includestapered threads 17 machined at the upper portion thereof for connectingthe conventional matrix-type drill bit 10 to a string of drill pipe (notshown). Machined tapered threads 17 are formed prior to attachment ofthe shank 16 to the blank 14; therefore, proper alignment of the shank16 with the blank 14 is critical.

FIG. 1C shows another conventional matrix-type drill bit 11 having aconventional shank 16 and illustrates the interface between the shank 16and bit body 23. Conventional matrix-type drill bit 11 includes aninternal bore 12 generally centered about the central longitudinal axis3 thereof. Shank 16 includes tapered threads 17 for attachment to adrill string (not shown) as well as “bit breaker” surface 21 forloosening and tightening the tapered thread connection between thematrix-type drill bit 11 and the drill string (not shown). Shank 16 maybe affixed to the bit body 23 by threaded connection 15 as well as weld20. Of course, bit body 23 includes a blank (not shown) that providesthe interfacing surface between the bit body 23 and the shank 16.

FIG. 1D shows a conventional steel body rotary drill bit 30 includingbit body 44 and internal bore 32 generally centered about central axis33. As FIG. 1D shows, conventional steel body rotary drill bit 30includes shank 36 having a threaded connection 37 for connecting to adrill string wherein the shank 36 is affixed by weld 40 to the bit body44. Bit body 44 may also carry blade(s) 42 having cutting elements 38for removing formation during subterranean drilling.

As may be seen in FIGS. 1C and 1D, in manufacturing either a matrix-typeor steel body rotary drill bit, a shank is affixed to a bit body. Inaddition, in conventional welding of a shank to a bit body of a rotarydrill bit, the shank may comprise a material having a carbon equivalentof higher than about 0.35%, such as, for example, an AISI 4140 steel.Therefore, the shank and bit body may be preferably preheated to about700° Fahrenheit before welding begins. Further, conventional weldingprocedures may designate that as the shank is welded to the bit body, ifthe temperature of the shank reaches 900° Fahrenheit the weldingprocedure may be interrupted until the temperature is reduced. When theconventional weld procedure resumes subsequent to delay caused by eitheroverheating or inadequate heating of the shank, the weld may continuefrom substantially the same circumferential position as occurred atinitiation of the delay.

U.S. Pat. No. 6,116,360 discloses, in discussing a prior art steelbodied bit, a shank welded to a steel bit body that protrudes therein.However, the mating surfaces between the shank and the steel bit bodyare not tapered.

In addition, U.S. Pat. No. 5,150,636 to Hill discloses a shrink-fitbetween a cutting head and a shank. Further, Hill discloses that the tipof the shank may have a slight reverse taper to better retain thecutting head.

It has been observed by the inventors herein that the conventionalthreaded connection between the shank and blank may generate undesirablestresses within the threaded joint and proximate weld joint. Inaddition, the conventional threaded connection may produce misalignmentbetween the shank and bit body. Further, it has been observed that aconventional single-pass weld between the blank and shank may allow oreven promote distortion and misalignment therebetween. Thus, it would beadvantageous to eliminate the need for preheating of the shank prior towelding the shank to the bit body and a need exists for an improvedshank configuration for use in fabricating rotary drill bits.

BRIEF SUMMARY OF THE INVENTION

The present invention provides a well-reasoned, practicallyimplementable shank configuration particularly suitable for rotary drillbits, which configuration may be tailored to specific bit sizes andarrangements. In the inventive shank configuration, the shank maycomprise, a material having a carbon equivalent that is less than about0.35%, for example, an AISI 4130 steel or AISI 4130MOD steel. Such aconfiguration may enable elimination of preheating prior to welding thatis typically required by conventional shank materials, such as AISI 4140steel, and the present invention contemplates and encompasses a methodof welding a shank structure to a portion of a bit body withoutpreheating of the shank structure.

Also according to the present invention, positioning of the shank inrelation to the bit body may be accomplished by engagement of taperedsurfaces thereof. For instance, a tapered surface or feature of theshank may be configured and sized to matingly engage a complementarilyshaped surface or feature of the drill bit body, such as on a portion ofa blank in the case of a matrix-type bit or any suitable portion of thebody in the case of a steel body bit, to become centered or positionedin relation thereto. The present invention is not limited to anyparticular tapered surface, since many arrangements may provide suchpositioning and more than one tapered surface may be employed. A taperedsurface or feature configuration may improve positioning of the blank inrelation to the shank, and also may eliminate conventional machining ofthreads therebetween. Exemplary tapered, complementary surfaces that maybe easily formed for implementation of the present invention includewithout limitation surfaces of revolution such as frustoconicalsurfaces, wherein such surfaces of revolution may be formed bymachining.

In addition, a multi-pass weld procedure may be employed whereinmultiple individual circumferential welds originate from differentcircumferential positions. Such a weld procedure and configuration mayalign or maintain alignment of the welded assembly of the shank with thebit body by equalizing or minimizing distortion caused by conventionalwelding processes. Put another way, a multi-pass weld may be formedwherein subsequent weld origination circumferential positions are offsetfrom immediately preceding weld origination circumferential positions.

For instance, a first weld may originate at a first position and extendaround the circumference of a weld recess to a second position. A secondweld may then be formed that originates from a substantially differentcircumferential position than the circumferential beginning point of thefirst weld. Subsequent welds, similarly, may be formed so that eachsubsequent weld originates at a circumferential position that differsfrom its preceding weld's originating position. In one embodiment, theoriginating position for a subsequent weld may be separated from thecircumferential origination position of its preceding weld by betweenabout 90° and about 180°.

It is specifically contemplated that the blank and shank configurationaccording to the present invention may be applied to coring bits,bi-center bits, eccentric bits, reaming tools and other drillingstructures as well as to full-bore drill bits. As used herein, the term“bit” encompasses all of the foregoing drilling structures, whethersteel or matrix-type. Moreover, the present invention is not limited toany particular structure for steel or matrix-type rotary drag bits andmay be applied to rotary drag bits fabricated by various methods. It isfurther contemplated that the blank and shank configuration according tothe present invention may be applied to fabrication of roller cone bits,and the term “bit” as used herein encompasses such assemblies.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1A is a perspective view of a conventional matrix-type rotary dragbit;

FIG. 1B is a partial schematic side cross-sectional view of theconventional matrix-type rotary drag bit shown in FIG. 1A;

FIG. 1C is a partial side cross-sectional view of a shank and bit bodyof a conventional matrix-type rotary drag bit;

FIG. 1D is a side cross-sectional view of a conventional steel bodyrotary drill bit;

FIG. 2A is a partial side cross-sectional view of a shank and rotarydrill bit body of the present invention;

FIG. 2B is a partial side cross-sectional view as well as a partial sideview of a shank and bit body of the present invention;

FIGS. 2C and 2D illustrate schematic top views of a multiple-pass weldand welding process of the present invention;

FIGS. 3A-3G are partial schematic side cross-sectional views ofdifferent embodiments of interface configurations between a bit body anda shank of the present invention;

FIG. 4 is a side view of a rotary drill bit according to the presentinvention;

FIG. 5A shows a perspective view of a rotary drill bit of the presentinvention; and

FIG. 5B shows a partial top cross-sectional view of a shank and bit bodyas shown in FIG. 5A.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 2A depicts a partial cross-sectional view of matrix-type rotarydrag bit 110 according to the present invention. Rotary drag bit 110includes central longitudinal axis 103 about which bore 112 is generallydisposed. Shank 116 may be comprised of a material having a carbonequivalent of less than about 0.35%, such as, for example and not by wayof limitation, an AISI 4130 steel or AISI 4130MOD steel and may includea threaded pin connection 117, as known in the art, for connection to adrill string (not shown) as well as a bit breaker surface 121 forassembly and disassembly thereto and therefrom, respectively. It may bedesirable for the shank material to have a carbon equivalent of evenless than about 0.35% such as, for example, less than about 0.30%. Itwill also be appreciated by those of ordinary skill in the art that thematerial selected for shank 116 exhibits, for example, at least aminimum yield strength, a minimum ultimate tensile strength and aminimum impact strength suitable for conditions encountered duringdrilling with rotary drag bit 110. The aforementioned AISI 4130 and AISI4130MOD steels possess such desirable mechanical properties.

Generally, a carbon equivalent is an empirical value in weight percentthat relates the combined effects of different alloying elements used inthe making of metal alloys, such as steels, to an equivalent amount ofcarbon, as an indication of weldability or susceptibility to weldcracking. A carbon equivalent may be used for hardenable carbon andalloy steels, without limitation. Further, as seen from the followingequation, it is not necessary that the material include carbon to have anon-zero carbon equivalent. Different formulas for computing a carbonequivalent of a material, as known in the art, have been developed. Thepresent invention contemplates use of different empirical formulas forcomputation of a carbon equivalent. For example, one formula for acarbon equivalent of a given material, provided from the MetalsHandbook®, Desk Edition, published by The American Society for Metals,eighth printing May, 1995, is given below.

${CE} = {{\%\mspace{14mu} C} + \frac{{\%\mspace{14mu}{Cr}} + {\%\mspace{14mu}{Mo}} + {\%\mspace{14mu} V}}{5} + \frac{{\%\mspace{14mu}{Si}} + {\%\mspace{14mu}{Ni}} + {\%\mspace{14mu}{Cu}}}{15}}$Where:CE is the carbon equivalent in weight percent;% C is the weight percent of carbon in the material;% Cr is the weight percent of chromium in the material;% Mo is the weight percent of molybdenum in the material;% V is the weight percent of vanadium in the material;% Si is the weight percent of silicon in the material;% Ni is the weight percent of nickel in the material; and% Cu is the weight percent of copper in the material.Thus, it will be appreciated that a material possessing desiredmechanical properties for use in shank 116 may be readily qualified interms of carbon equivalent as to its suitability for use inimplementation of the present invention.

In addition, shank 116 may also include tapered surface 160 configuredto matingly engage complementary tapered surface 161 of bit body 123,thus positioning shank 116 with respect to bit body 123 and forming, incombination with tapered surface 141 of bit body 123, weld recess 139.By way of example only, and as applicable to this and the otherillustrated embodiments of the present invention, the referenced taperedsurfaces may, but do not necessarily have to be, implemented asfrustoconical surfaces. Vertical surface 150 of shank 116 may extendwithin bit body 123 along vertical surface 151 of bit body 123, but maybe configured with tapered surface 160 to position shank 116 withrespect to bit body 123. FIG. 2A also shows that horizontal surface 140radially inward of tapered surface 160 may be separated from horizontalsurface 152 of bit body 123 by gap 111 to prevent contact therebetween,because such contact may affect position of shank 116 in relation to bitbody 123, notwithstanding mutual contact of tapered surface 160 andtapered surface 161. As noted above, radially outermost portions oftapered surface 160 and tapered surface 161 together definecircumferential weld recess 139 wherein a weld 170, such as a multi-passweld according to the present invention, may be formed.

FIG. 2B shows a partial side cross-sectional view of a rotary drill bit310 (left-hand side of figure) and a partial side view of rotary drillbit 310 about its longitudinal axis 333 (right-hand side of figure)prior to welding in accordance with the present invention. Rotary drillbit 310 may generally comprise a bit body 323 including a plurality oflongitudinally extending blades 314 defining junk slots 316therebetween. Each blade 314 may define a leading or cutting face 318that extends radially along the bit face around the distal end 315 ofthe rotary drill bit 310, and may include a plurality of cutting elementpockets 319 formed within bit body 323 and oriented for affixing cuttingelements 320 therein to cut a subterranean formation upon rotation ofthe rotary drill bit 310. Cutting elements 320 are shown forillustration only, as they may be affixed to the cutting element pockets319 after the shank 334 is welded to the bit body 323. Shank 334,according to the present invention, may comprise a material having acarbon equivalent of less than about 0.35%, such as an AISI 4130 or AISI4130MOD steel. Each blade 314 may include a longitudinally extendinggage portion 322 that corresponds to the gage 312 of each blade 314,sized according to approximately largest-diameter portion of the rotarydrill bit 310 and thus may be typically only slightly smaller than thediameter of the hole to be drilled by rotary drill bit 310. The upperlongitudinal end 317 of the rotary drill bit 310 includes a threadedportion or pin 325 to threadedly attach the rotary drill bit 310 to adrill collar or downhole motor, as is known in the art. In addition, abore or plenum 329 longitudinally extends within rotary drill bit 310for communicating drilling fluid therewithin through nozzles 336disposed on the face of the rotary drill bit 310 through passages (notshown) extending from plenum 329 to nozzles 336. Threaded portion 325may be machined directly into the upper longitudinal end 317 of theshank 334, as may bit breaker surface 321, for loosening and tighteningthe tapered threaded portion 325 of the rotary drill bit 310 wheninstalled into the drill string, the shank 334 engaging the bit body 323of the rotary drill bit 310 at its distal end 315 as depicted in thecross-sectional view thereof.

Also as shown in FIG. 2B, tapered surface 350 of the shank 334 maymatingly engage tapered surface 351 of the bit body 323 in order toposition the shank 334 in relation to the bit body 323. Of course,vertical surface 360 of shank 334 may engage vertical surface 361(vertical surfaces 360 and 361 not necessarily being threaded asdepicted in FIG. 2B) and horizontal surface 370 of shank 334 may or maynot engage horizontal surface 371 according to actual clearancestherebetween, the desirability of a gap being heretofore described withrespect to FIG. 2A. Weld recess 339 may be formed by tapered surface 350of shank 334 and tapered surface 352 of the bit body 323.

A multi-pass weld of the present invention, as described hereinbelow,may be formed and disposed generally within weld recess 339. As notedabove, shank 334 may comprise a material having a carbon equivalent ofless than about 0.35%, such as, for example, an AISI 4130, an AISI4130MOD steel, or an equivalent material. Therefore, preheating shank334 prior to initiating the welding process may not be necessary. As afurther advantage, aligning the shank 334 with respect to the bit body323 and then tack-welding the assembly together may be accomplished.FIG. 2C shows a schematic top cross-sectional view of multi-pass weld401 of the present invention in relation to the inner apex or tip 340 ofthe weld recess 339 as shown in FIG. 2B. More particularly, FIG. 2Cshows a top view of the inner boundary of weld recess 339, as defined bytip 340 thereof, as well as welds 410, 420, 430, and 440. Welds 410,420, 430, and 440 are depicted as concentric rings or circles ofincreasing diameter and are shown as being separate from one another.However, FIG. 2C is merely schematic, and welds 410, 420, 430, and 440are depicted as shown merely for clarity. Welds 410, 420, 430, and 440may be disposed anywhere generally within weld recess 339, depending onthe size of the previous welding passes and the size of the weld recess339. Of course, the longitudinal position of any weld of the presentinvention may be varied in order to fill the weld recess relativelyevenly.

As shown in FIG. 2C, a first weld 410 or “root” weld may be depositedwithin the weld recess 339, or more specifically, positioned along thecircumference of tip 340 of the weld recess 339 formed by the interfacebetween the facing surfaces 350 and 352 of the shank 334 and the bitbody 323. First weld 410 may extend around the circumference of tip 340of weld recess 339. First weld 410, as shown in FIG. 2C, may originateat circumferential position 409 and may also terminate thereat.Alternatively, first weld 410 may originate at a first circumferentialposition and may terminate at a second circumferential position. Secondweld 420, as shown in FIG. 2C, may originate at circumferential position419 and may terminate thereat. Alternatively, second weld 420 mayoriginate at a first circumferential position separated from thecircumferential origination position of the first weld 410 by at leastabout 90° and may terminate at a second circumferential position.

Therefore, circumferential position 409 may be separated fromcircumferential position 419 by at least about 90°, measured in relationto the longitudinal axis 333 of rotary drill bit 310, either in theclockwise or counter-clockwise direction. Separation angle θ, shown byFIG. 2C, illustrates such a measure of separation betweencircumferential position 409 and circumferential position 419. Further,second weld 420 may originate at a first circumferential positionseparated from the originating circumferential position of theimmediately preceding weld by at least about 90°, and may terminate at asecond circumferential position. In addition, second weld 420 may beformed about longitudinal axis 333 in a circumferential direction(clockwise or counter-clockwise) opposite to or consistent with thedirection that the first weld 410 was formed.

Third weld 430, as shown in FIG. 2C, originates at circumferentialposition 429 and also terminates thereat. More generally, third weld 430may originate at a first circumferential position separated from theoriginating circumferential position of the immediately preceding weldby at least about 90°, and may terminate at a second circumferentialposition. Fourth weld 440, as shown in FIG. 2C, originates atcircumferential position 439 and also terminates thereat. Similarly,fourth weld 440 may originate at a first circumferential positionseparated from the originating circumferential position 429 of theimmediately preceding third weld 430 by at least about 90°, and mayterminate at a second circumferential position. As may also be seen fromFIG. 2C, originating circumferential positions 409, 419, 429, and 439may be substantially symmetrically distributed about the circumferenceof tip 340 of weld recess 339.

Of course, the separation between an originating position of a precedingweld and the originating position of a subsequent weld may be measuredin relation to the circumferential distance therebetween. For instance,the circumferential separation distance between circumferential position409 and circumferential position 419 may be at least about one quarterof the circumference of the circle depicting tip 340 of weld recess 339.

Therefore, a multi-pass weld of the present invention may include aninitial weld originating at a first circumferential position andterminating at a second circumferential position and a second weldoriginating at a circumferential position at least about 90° from thefirst position of the first weld or at least about one quarter of thecircumference of the tip 340 of weld recess 339. Subsequent welds mayoriginate at respective circumferential positions that are separated byat least about 90° from the circumferential originating position oftheir immediately preceding weld or a distance of at least about onequarter of the circumference of the tip 340 of weld recess 339,therearound, respectively. Circumferential positions may only beseparated by up to 180°, since such positioning would be on oppositesides of a line from one edge of the circumference through the centerthereof to the other side of the circumference. Thus, subsequent weldsmay originate at respective circumferential positions that are separatedfrom the originating position of the immediately preceding weld by about90° to 180° from the originating position of the immediately precedingweld in accordance with the present invention. Such a weld configurationmay reduce, equalize, or minimize distortion and misalignment betweenthe assembled shank 334 and bit body 323.

As a further example of the multi-pass weld of the present invention,and without limitation, FIG. 2D shows a top cross-sectional view ofmulti-pass weld 402 in relation to the tip 340 of the weld recess 339 asshown in FIG. 2B. Welds 452, 454, 456, 458, 460, and 462 may be formedand extend around the circumference of the tip 340 of weld recess 339.First weld 452 may originate at circumferential position 453 and mayalso terminate thereat. Second weld 454 may originate at circumferentialposition 455 and may also terminate thereat. Third weld 456, mayoriginate at circumferential position 457 and may also terminatethereat. Fourth weld 458, may originate at circumferential position 459and may also terminate thereat. Fifth weld 460, may originate atcircumferential position 461 and may also terminate thereat. Sixth weld462, may originate at circumferential position 463 and may alsoterminate thereat.

Alternatively, and more generally, each weld 452, 454, 456, 458, 460,and 462 may originate at a first circumferential position that is offsetfrom or separated from the circumferential origination position of itspreceding weld. Thus, subsequent welds 454, 456, 458, 460, and 462,meaning welds that occur after a preceding weld, may originate at acircumferential position separated from the originating circumferentialposition of the immediately preceding weld by at least about 90° or atleast about one quarter of the circumference of weld tip 340. Forinstance, separation angle θ, shown by FIG. 2D, illustrates a measure ofseparation between circumferential position 459 and circumferentialposition 463 of about 120°. Put another way, the separation distancebetween circumferential position 459 and circumferential position 463 asshown in FIG. 2D is about one third of the circumference of the circledepicting tip 340 of weld recess 339. Further, welds 452, 454, 456, 458,460, and 462 may be formed about longitudinal axis 333 along anycircumferential direction (clockwise or counter-clockwise).

Thus, the multi-pass weld of the present invention is not limited to anyparticular number of discrete welds, but rather comprises more than oneweld wherein the origination position of a preceding and subsequent weldis separated by at least about 90° or at least about one quarter of thecircumference of tip 340 of weld recess 339. Further, the welds may ormay not extend circumferentially or at all. For instance, the welds maybe formed by applying a heat source and welding medium at a particularposition, forming a weld and then positioning the heat source andwelding medium at a second position and forming another weld. Thus,welds may be formed within a weld recess at discrete locations. Inaddition, the separation between the circumferential position oforigination between a preceding and immediately subsequent weld mayvary. For instance, the separation angle θ may be about 90°, then about135°, then about 180°, for the second weld, the third weld, and thefourth weld, respectively, without limitation. Further, the originationpositions of the welds may form a substantially symmetrical pattern, ormay form an unsymmetrical pattern.

FIG. 3A shows a partial cross-sectional view of an interface 200 betweena shank 216 and a bit body 223 with respect to bore 212 centered aboutcentral axis 203 of a rotary drag bit (remainder not shown). Shank 216may comprise a material having a carbon equivalent of less than about0.35% and may include tapered surface 260, tapered surface 250, andhorizontal surface 253. Tapered surface 250 of shank 216 may beconfigured to matingly engage tapered surface 251 of bit body 223 toposition shank 216 with respect to bit body 223. Further, gap 211 mayseparate horizontal surface 253 of shank 216 and horizontal surface 252of bit body 223, thus inhibiting engagement therebetween that may affectthe proper mating engagement between tapered surface 250 of shank 216and tapered surface 251 of bit body 223. Weld recess 239 may be formedby the intersection of tapered surface 260 of shank 216 with taperedsurface 241 of bit body 223. As may be further seen in FIG. 3A, taperedsurface 251 and horizontal surface 252 of bit body 223 may form a cavitywhich the lower longitudinal end of shank 216 fits within. Such aconfiguration may be advantageous for distributing stresses transmittedthrough the shank 216 during operation of the rotary drag bit.

Alternatively, gap 211 may be reduced or eliminated by way of alongitudinal force applied to compress the bit body 223 and the shank216 against one another. Stated another way, it may be desirable toconfigure tapered surface 250 of shank 216 and tapered surface 251 ofbit body 223 so that a sufficient compressive force causes slidingtherebetween, reducing gap 211 or causing horizontal surface 253 ofshank 216 to engage horizontal surface 252 of bit body 223. Such acompressive force may be applied prior to or during welding of the shank216 to the bit body 223, or both, and may be desirable as generating atensile residual stress within the multi-pass weld (FIGS. 2C and 2D)that may be counter-acted by the compressive forces experienced duringdrilling. Such a configuration may reduce the stresses in the weldduring drilling. As a further alternative, gap 211 may be eliminated bysizing the shank 216 and bit body 223 accordingly.

FIG. 3B shows a partial cross-sectional view of another embodiment ofinterface 201 between shank 216 and bit body 223 with respect to bore212 centered about central axis 203 of a rotary drag bit (remainder notshown). Shank 216 may comprise a material having a carbon equivalent ofless than about 0.35%, such as, for example, an AISI 4130 or AISI4130MOD steel and may include tapered surface 260, tapered surface 250,and horizontal surface 253. Tapered surfaces 250 and 260 of shank 216may be configured to matingly engage tapered surfaces 251 and 261 of bitbody 223, respectively, to position shank 216 with respect to bit body223. Such a dual-taper configuration may be advantageous for positioningthe shank 216 with respect to the bit body 223.

Further, gap 211 may separate horizontal surface 253 of shank 216 andhorizontal surface 252 of bit body 223, thus inhibiting engagementtherebetween that may affect the proper mating engagement betweentapered surfaces 250 and 260 of shank 216 and tapered surfaces 251 and261 of bit body 223, respectively. Weld recess 239 may be formed by theengagement of tapered surface 260 of shank 216 with tapered surface 241of bit body 223. As may be further seen in FIG. 3B, tapered surface 251and horizontal surface 252 of bit body 223 may form a cavity which thelower longitudinal end of shank 216 fits within. Such a configurationmay be advantageous for distributing stresses transmitted through theshank 216 during operation of the rotary drag bit (not shown).

FIG. 3C shows a partial cross-sectional view of another embodiment ofthe present invention depicting interface 202 between shank 216 and bitbody 223 with respect to bore 212 centered about central axis 203 of arotary drag bit (not shown). As shown in FIG. 3C, tapered surface 270 ofshank 216 may be sloped longitudinally downward along a radially inwardpath, and may matingly engage tapered surface 271 of bit body 223, whichmay slope longitudinally upward along a radially outward path. Thus,mating engagement between tapered surface 270 of shank 216 and taperedsurface 271 of bit body 223 may position shank 216 with respect to bitbody 223. Weld recess 239 may be substantially formed by theintersection of tapered surface 241 of bit body 223 and tapered surface270 of shank 216. Of course, chamfers and radii at boundaries betweenadjacent surfaces may be used in accordance with engineering design tofacilitate proper engagement between tapered surface 270 of shank 216and tapered surface 271 of bit body 223. As may also be seen inreference to FIG. 3C, tapered surface 271 of bit body 223 may form acavity which the lower longitudinal portion of shank 216 fits within.Such a configuration may be advantageous for distributing stressesduring operation of the rotary drag bit (not shown). In addition, shank216 may comprise a material having a carbon equivalent of less thanabout 0.35%, in order to eliminate the need for preheating prior towelding of the shank 216 to the bit body 223.

FIG. 3D shows a partial cross-sectional view of yet another embodimentof the present invention depicting interface 204 between shank 216 andbit body 223 with respect to bore 212 centered about central axis 203 ofa rotary drag bit (remainder not shown). As shown in FIG. 3D, taperedsurface 280 of shank 216 may slope longitudinally upward along aradially inward path, and may matingly engage tapered surface 281 of bitbody 223, which may slope longitudinally downward along a radiallyoutward path. Thus, mating engagement between tapered surface 280 ofshank 216 and tapered surface 281 of bit body 223 may position shank 216with respect to bit body 223.

Weld recess 239 may be substantially formed by the intersection oftapered surface 282 of shank 216 and tapered surface 280 of bit body223. Shank 216 may comprise a material having a carbon equivalent ofless than about 0.35%, such as, for example, an AISI 4130 steel, an AISI4130MOD steel, or an equivalent material, to eliminate the need forpreheating the shank prior to welding the shank 216 and bit body 223 toone another. Such a configuration may allow the shank 216 and bit body223 to be tack welded in order to maintain the relative positioningthereof prior to forming the multi-pass weld as described above andeliminate conventional preheating thereof during welding.

FIG. 3E shows a further embodiment of the present invention depicting across-sectional view of interface 205 between shank 216 and bit body223. As mentioned above, shank 216 may comprise a material having acarbon equivalent of less than about 0.35%. Shank 216 and bit body 223are shown in relation to bore 212, which is centered about central axis203 of a rotary drag bit (remainder not shown). Tapered surface 292 ofshank 216 may matingly engage tapered surface 293 of bit body 223 toposition the shank 216 in relation to the bit body 223. Also, horizontalsurface 300 of shank 216 may matingly engage horizontal surface 295 ofbit body 223, thereby vertically positioning the shank 216 in relationto the bit body 223. Gap 299 may exist between tapered surface 290 ofshank 216 and tapered surface 297 of bit body 223. Gap 299 may provideclearance for fitting the shank 216 and the bit body 223 together. Weldrecess 239 may be substantially formed by tapered surface 290 of shank216 and tapered surface 291 of bit body 223.

FIGS. 3F and 3G show a cross-sectional view of interface 206 accordingto the present invention between shank 216 and bit body 223. Morespecifically, a deformable element 302 may be positioned between shank216 and bit body 223. As shown in FIGS. 3F and 3G, deformable element302 may be positioned between horizontal surface 304 of shank 216 andhorizontal surface 305 of bit body 223. Gap 311 may exist initiallybetween tapered surface 306 of shank 216 and tapered surface 307 of bitbody 223. Further, tapered surface 303 of bit body 223 may engagetapered surface 308 of shank 216 or, alternatively, there may be slightclearance therebetween. However, as shown in FIG. 3G, shank 216 may bedisplaced so as to deform deformable element 302 and position taperedsurface 306 of shank 216 to matingly engage tapered surface 307 of bitbody 223, thus substantially eliminating gap 311. Such a configurationmay be preferable to position the shank 216 in relation to the bit body223 by way of a compressive force. Such a compressive force may beapplied prior to and/or during welding of the shank 216 to the bit body223, and may effect a tensile residual stress within the multi-pass weld(FIGS. 2C and 2D) that may be desirable as reducing the stresses in theweld during drilling. Also, as shown in FIGS. 3F and 3G, weld recess 339may be substantially formed by tapered surface 306 of shank 216 andtapered surface 301 of bit body 223. Exemplary deformable elements 302include high temperature elastomeric rings, annular leaf springs andBelleville springs, as well as nonresilient deformable materials thatmay be crushed as gap 311 is eliminated. Deformation, resilient ornonresilient, of deformable element 302 may provide controlled downwardmovement of shank 216 as it is caused to engage bit body 223.

FIG. 4 shows an exemplary rotary drag bit 500 according to the presentinvention wherein an interface and multi-pass weld as described abovehave been completed to affix bit body 323, either steel body ormatrix-type, to the shank 334. Shank 334 may include bit breakersurfaces or flats 321 for loosening and tightening the tapered threadedportion 325 of the rotary drag bit 500 when installed into the drillstring (not shown). Rotary drag bit 500 may include radially andlongitudinally extending blades 314, wherein each blade 314 may define aleading or cutting face 318 and may include a plurality of cuttingelements 320 affixed thereto and oriented therein to cut a subterraneanformation upon rotation of the rotary drag bit 500. Nozzles 336 may besized and positioned to communicate drilling fluid from the interior ofthe rotary drag bit 500 to the cutting elements 320 and blades 314 toclean cuttings therefrom. Upon completion of multi-pass weld (notshown), the exterior, radially outward surface thereof may be machinedflush with an outer surface of bit body 323. Further, it should beunderstood that the present invention is not limited to rotary-drillbits fabricated by way of any particular method; rather, the presentinvention may be practiced with rotary drill bits fabricated by anymethod.

Generally, the tapered surface arrangements and configurations of thepresent invention may provide an efficient mechanism to position theshank in relation to the bit body in preparation for weldingtherebetween. In addition, a longitudinal, generally axial force may beapplied to the shank or bit body as described hereinabove to facilitatepositioning or centering of the shank in relation to the bit body, withor without the disposition of a deformable element therebetween. Also, alongitudinal force may be applied to achieve a desired stress state inthe assembly in relation to welding the shank to the bit body. Thelongitudinal force may be applied externally, by way of a piston or byother force generation means. On the other hand, with respect only topositioning, the tapered surfaces of the shank and bit body may beconfigured and sized so that the weight of the shank as it is disposedlongitudinally above the bit body facilitates positioning or centeringthereof in relation to the bit body as it is lowered thereonto. In sucha configuration, the shank may be “self-centering.”

In addition, although the foregoing descriptions depict “taperedsurfaces” in the form of cross-sectional representations that may implycontinuous annular surfaces such as frustoconical surfaces, the presentinvention contemplates that the tapered surfaces may comprise moregenerally tapered features that may or may not be continuous and may ormay not be linear in cross-section. Likewise, although the foregoingillustrations and descriptions may imply an annular weld recess, manyalternatives are contemplated by the present invention. For instance,the multi-pass weld of the present invention may be formed in relationto, generally, a region configured for forming a welded connectionbetween the shank and bit body, without limitation.

More specifically, the present invention contemplates that complementarylongitudinal recesses may be formed in the mating ends of both the shankand bit body for welding to one another. In other words, thelongitudinal mating ends of both the shank and bit body may comprisesplines that may be aligned to form longitudinal weld recesses. In sucha configuration, a respective weld may be formed within each alignedlongitudinal weld recess. However, in such a configuration, themulti-pass weld of the present invention may be formed within thelongitudinal weld recesses. More specifically, in such a configuration,a first weld may originate from a first circumferential position and asecond weld may originate from a circumferential position separated fromthe first circumferential position. Each subsequent weld may originatefrom a respective circumferential position that is at least about 90°from the origination position of its immediately preceding weld.

FIG. 5A shows a perspective view of a rotary drill bit 610 prior towelding in accordance with the present invention. Rotary drill bit 610may generally comprise a bit body 623 including a plurality oflongitudinally extending blades 614 defining junk slots 616 therebetweenand having a leading or cutting face 618 that extends radially along thebit face of the rotary drill bit 610. Bit body 623 may include aplurality of cutting elements 620 affixed thereto to cut a subterraneanformation upon rotation of the rotary drill bit 610. Cutting elements620 are shown for illustration only, as they may be affixed to the bitbody 623 after the shank 634 is welded to the bit body 623, inaccordance with conventional practices. Shank 634, according to thepresent invention, may comprise a material having a carbon equivalent ofless than about 0.35%. For example, an AISI 4130 steel, an AISI 4130MODsteel, or any material having a carbon equivalent of less than about0.35% may be used, without limitation. Each blade 614 may define alongitudinally extending gage portion 622 that corresponds to the gage612 of each blade 614, sized according to approximately largest-diameterportion of the rotary drill bit 610. The upper longitudinal end 617 ofthe rotary drill bit 610 includes a threaded portion or pin 625 tothreadedly attach the rotary drill bit 610 to a drill string (notshown), as is known in the art. In addition, drilling fluid may becommunicated through nozzles 636 disposed on the face of the rotarydrill bit 610.

Shank 634 includes longitudinal recesses 650 which correspond tolongitudinal recesses 660 of bit body 623. Further, shank 634 mayinclude a tapered feature 670, which may be configured according to anyof the embodiments described in FIGS. 2A, 2B, and 3A-3G, and which maybe termed a protrusion for the sake of convenience only. Of course, bitbody 623 may include a complementary tapered feature (not shown), whichmay be termed a recess for the sake of convenience only. Upon assemblyof shank 634 and bit body 623, the longitudinal recesses 650 of theshank 634 and the longitudinal recesses 660 of the bit body 623 may bealigned circumferentially. FIG. 5B shows a partial top cross-sectionalview of the longitudinal recesses 650 of the shank 634 (FIG. 5A) and thelongitudinal recesses 660 of the bit body 623 (FIG. 5A) wherein thelongitudinal recesses 650 and 660 are vertically superimposed andcircumferentially aligned. Such alignment may form weld recesses 655, asshown in FIG. 5B.

Further, according to the present invention, a multi-pass weld may beformed within weld recesses 655. A first weld 680 is shown in FIG. 5B ata circumferential position of origination 682, and may extendlongitudinally within the aligned longitudinal recesses 650 and 660. Asecond weld 681 may be formed at a circumferential position oforigination 683 that is separated from the circumferential position oforigination 682 of first weld 680 by at least 90°, as depicted byseparation angle θ in relation to longitudinal axis 661. Subsequentwelds (not shown) may be positioned so that each subsequentcircumferential position of origination is separated from thecircumferential position of origination of its immediately precedingweld.

There are many alternative implementations that are contemplated andencompassed by the present invention. For instance, a weld region may beformed by alignment of spiraled splines or recesses in one or both ofthe shank and bit body. Further, although the multi-pass weld of thepresent invention may be described in terms of preceding and subsequentwelds, as hereinabove, it is contemplated that one or more welds of thepresent invention may be formed substantially simultaneously by way ofapplication of multiple heat sources and disposition of weldingmaterials at more than one location within a weld region. In such aconfiguration, a simultaneously formed weld may be taken as eithersubsequent or preceding in relation to any other weld simultaneouslyformed therewith, without limitation. For example, without limitation,the present invention contemplates that two welds may be formedsubstantially simultaneously, separated by a separation angle of atleast about 90°. Further, for example, without limitation, the presentinvention contemplates that three welds may be formed substantiallysimultaneously, wherein at least two of the three welds are separated byat least about 90°. Such a configuration may increase the cost of thewelding equipment, but may also increase the speed or performance of thewelding process and further reduce any tendency toward misalignment ofthe shank and bit body that may be induced by welding.

While the present invention has been described herein with respect tocertain preferred embodiments, those of ordinary skill in the art willrecognize and appreciate that it is not so limited. Rather, manyadditions, deletions and modifications to the preferred embodiments maybe made without departing from the scope of the invention as hereinafterclaimed. In addition, features from one embodiment may be combined withfeatures of another embodiment while still being encompassed within thescope of the invention as contemplated by the inventors. Further, theinvention has utility in drill bits and core bits having different andvarious bit profiles as well as cutter types.

What is claimed is:
 1. A shank structure for a rotary drill bit forsubterranean drilling, comprising: at least one feature for engaging acomplementary feature of a bit body of a rotary drill bit; a trailingend having structure associated therewith for connecting the rotarydrill bit to a drill string; and wherein the shank structure comprises amaterial differing at least in part from at least one material of thebit body and having a carbon equivalent of less than about 0.35%.
 2. Theshank structure of claim 1, wherein the carbon equivalent is less thanabout 0.30%.
 3. The shank structure of claim 1, wherein the shankstructure comprises AISI 4130 steel or AISI 4130MOD steel.
 4. The shankstructure of claim 1, wherein the at least one feature for engaging acomplementary feature of a bit body of a rotary drill bit comprises atapered feature for matingly engaging the complementary feature of thebit body of the rotary drill bit and at least in part positioning theshank structure in relation to the bit body of the rotary drill bit. 5.The shank structure of claim 4, wherein the tapered feature comprises atapered surface.
 6. The shank structure of claim 1, wherein the at leastone feature for engaging a complementary feature of a bit body of arotary drill bit comprises at least one frustoconical feature formatingly engaging a complementary frustoconical feature of the bit bodyof the rotary drill bit.
 7. The shank structure of claim 6, furthercomprising at least one weld in contact with a portion of the at leastone frustoconical feature of the shank structure.
 8. The shank structureof claim 7, wherein the at least one weld comprises first and secondwelds formed substantially simultaneously.
 9. The shank structure ofclaim 7, wherein the at least one weld comprises a multi-pass weldexhibiting a stress state at least in part responsive to a force appliedbetween the at least one frustoconical feature of the shank structureand the complementary frustoconical feature of the bit body duringformation of the multi-pass weld.
 10. The shank structure of claim 1,wherein a portion of the shank structure is configured for dispositionwithin a cavity formed within the bit body of the rotary drill bit. 11.The shank structure of claim 10, wherein the at least one complementaryfeature of the bit body forms at least a portion of the cavity.
 12. Theshank structure of claim 1, further comprising a deformable elementdisposed on the shank structure.